1. Field of the Invention
The present invention relates to an optical sheet member and an image display device employing the same.
2. Description of the Related Art
A flat panel display such as a liquid crystal display device (hereinafter, also referred to as an LCD) has been widely used as an image display device having small power consumption and space saving properties over the years. The liquid crystal display device has a configuration in which backlight (hereinafter, also referred to as BL), a backlight side polarizing plate, a liquid crystal cell, a visible side polarizing plate, and the like are disposed in this order.
In the recent flat panel display market, development for power saving, high definition, and color reproducibility improvement has progressed as an LCD performance enhancement, and currently, power saving, high definition, and color reproducibility improvement have been remarkably required in a small-size device, particularly for example, a tablet PC or a smart phone, but development of next generation high vision (4K2K, an EBU ratio of greater than or equal to 100%) of current TV standards (FHD, National Television System Committee (NTSC) ratio of 72%≅European Broadcasting Union (EBU) ratio of 100%) has progressed in a large-size device. For this reason, the power saving, the high definition, and the color reproducibility improvement of the liquid crystal display device have been increasingly required.
An optical sheet member is disposed between the backlight and the backlight side polarizing plate according to power saving of the backlight, and the optical sheet member is an optical device in which, among incident light rays while vibrating in all directions, only light rays vibrating in a specific polarization direction are transmitted, and light rays vibrating in the other polarization direction are reflected. As a core component of a low power LCD according to an increase in a mobile device and low power consumption in a home electric appliance, improvement of light efficiency of the LCD and an increase in brightness (a degree of brightness per unit area of a light source) have been expected.
In response, a technology has been known in which an optical sheet member (a Dual Brightness Enhancement Film (DBEF) or the like) is combined between the backlight and the backlight side polarizing plate, and thus a light utilization rate of the BL is improved according to optical recycling, and the brightness is improved while saving power of the backlight (refer to JP3448626B). Similarly, in JP1989-133003A (JP-H01-133003A), a technology is disclosed in which a light utilization rate of the BL is improved in the optical recycling according to broadband in a polarizing plate having a configuration in which a λ/4 plate and a cholesteric liquid crystalline phase are laminated and a layer formed by fixing cholesteric liquid crystalline phases of three or more layers having different pitches are laminated and a layer formed by fixing cholesteric liquid crystalline phases of three or more layers having different pitches of the cholesteric liquid crystalline phases.
However, such an optical sheet member has a complicated member configuration, and thus it is necessary to reduce the cost by reducing the number of members in which functions of the members are further integrated in order to spread the optical sheet member to the market.
On the other hand, a method has been known in which a light emitting spectrum of the backlight is sharpened from a viewpoint of high definition of a liquid crystal display device and of improving color reproducibility thereof. For example, in JP2012-169271A, a method is disclosed in which white light is realized by using a quantum dot (QD) emitting red light and green light as a fluorescent body between a blue LED and a light guide plate, and thus high brightness and an improvement in color reproducibility are realized. In SID'12 DIGEST p. 895, a method is proposed in which an optical conversion sheet using a quantum dot (QDEF, also referred to as a quantum dot sheet) is combined in order to enhance color reproducibility of the LCD.